"There is no reason to have this base here," said Antonio Faitta, 25, who travelled from Genoa for the protest.

Special trains and buses from various parts of Italy arrived in Vicenza for the march.

People want to impose with violence a base that nobody wants

Local man

Many of them had been chartered by leftist parties and the Greens, members of Mr Prodi's ruling coalition, although the prime minister had banned ministers from attending the march.

The BBC's David Willey in Rome says the centre-left government is embarrassingly split between those who want to respect the decision of the previous centre-right coalition to agree to Washington's request, and those who would like to see the Americans out.

Thousands set off from the town's railway station with banners such as "No To The Bases" and "America No Thanks".

"We love our town and we want to protect it," a local protester told the BBC.

"Other people want to impose with violence a base that nobody wants."

Transfer from Germany

The Americans established a military presence in Vicenza more than half a century ago.

President George W Bush wants to strengthen the base, the headquarters of the 173rd Airborne Brigade, by transferring from Germany to Italy another 2,000 US soldiers.

This would bring the total number of US troops stationed in Vicenza to nearly 5,000.

The base provides over 1,000 jobs to locals in Vicenza and injects millions of dollars into the local economy.